Inkjet printers, including color inkjet printers, are well-known. Inkjet printers incorporate one or more printheads in a scanning carriage. The printheads are typically housed in one or more print cartridges either containing ink or having ink supplied to them from an external source. The ink is channeled to vaporization chambers formed in a substrate associated with each printhead. Within each vaporization chamber is an ink ejection element, such as a resistive heater or a piezoelectric element. A nozzle plate resides over each printhead such that each nozzle is aligned over a respective vaporization chamber. Each printhead may have hundreds of nozzles formed therein for printing 300 or more dots per inch (dpi). As the scanning carriage scans across a printing medium from left to right and back, energization signals are provided to the ink ejection elements and the nozzles eject droplets of ink onto the printing medium to produce a printed image.
Typically, the scanning carriage of an inkjet printer scans across the printing medium several times to complete a swath of ink. Multiple passes of the scanning carriage are preferred to a single pass for several reasons. For example, a defective nozzle or ink ejection element would result in a white horizontal line across the medium. A single pass depositing all the ink needed for the image may provide too much ink in too short of a time to be absorbed by the medium. This would result in excessive ink bleed, excessive drying times, and cockling (warping) of the medium. Also, a single pass may not be sufficient to provide the desired color saturation. For at least these reasons, high quality inkjet printers use multiple passes, when appropriate, such that only a fraction of the total ink required for the image is deposited in a single pass, and any areas not covered by the first pass are filled by one or more later passes. Multiple pass techniques in an inkjet printer have been described in U.S. Pat. Nos. 5,555,006, 5,476,958, 5,276,467 and 4,965,593, which are assigned to the present assignee and incorporated herein by reference.
One problem with conventional inkjet printers is ink droplet or dot displacement. This problem is most apparent when printing a vertical line. Typical print cartridges cycle through their fire order only once per pixel. Since print cartridges continuously proceed through their fire order as the scanning carriage moves across the medium, ink droplets ejected from nozzles at the beginning of the fire order are deposited at their desired location, while those ejected at the end of the fire order are displaced from their desired position by a distance equal to the pixel width. For a 600 dpi printer, this error distance is 42 microns. Thus, a resulting vertical line will appear jagged rather than straight.
One solution to the dot displacement problem is to stagger the physical position of the nozzles and their respective vaporization chambers on the substrate of the printhead. Although effective at solving the dot displacement problem, this approach is relatively complex. The ink flow distance from the edge of the substrate to a vaporization chamber varies depending on the location of the particular vaporization chamber. Vaporization chambers located closer to the edge refill faster than those further away. This creates differences in both the volume and velocity of ejected ink droplets.
Another solution to the dot displacement problem involves rotating the entire substrate. This approach, however, employs a more complex print cartridge and scanning carriage in order to create the rotation. In addition, this print cartridge is more difficult to code and requires additional memory, since data for many different columns must be buffered up simultaneously.
Still another approach is minimizing dot displacement error by increasing the number of times per pixel that a print cartridge with non-staggered nozzles cycles through its fire order. A different problem, however, called horizontal banding exists. Visible horizontal bands result from repetitive variations in row densities due to positional errors in the displacement of ink droplets. Horizontal bands are more apparent among multiple pass printers that do not compensate for dot displacement with staggered nozzles than among those that do.
FIG. 1 illustrates the problem of horizontal banding. Here, a swath of ink has been deposited by a 600 dpi printer in a two-pass printing operation. The print cartridge of this printer, which cycles through its fire order four times per pixel, has non-staggered nozzles and a primitive size of eight. Each of the eight address lines of the print cartridge has a characteristic dot displacement error, which increases from address line 1 to address line 8. In FIG. 1 there is a visible horizontal band in rows 13-16 and 29-32. These bands result from a mismatch between the number of rows which the media is advanced and the primitive size. Because of the mismatch, the odd columns of each row are formed by nozzles associated with address lines different from those which form the even columns. Here, the printer advances the medium by twenty rows, and the primitive size is eight. The odd columns of rows 13 and 14 are printed by nozzles associated with address line 7, while the even columns are printed by nozzles associated with address line 1. Since the dot displacement error differs for the two address lines, with the error of address line 7 being greater than that of address line 1, the spacing between adjacent dots along the row varies, creating a distracting horizontal band.
There is a need, therefore, for a simple, high speed printer that reduces dot displacement error and horizontal banding.